What are tie-rod ends and boots?

Tie-rods and tie-rod end ball joints are what connect the steering rack gearbox to the steering knuckles so the turning input from the driver makes its way to the wheels. Tie-rods thread onto the steering rack to also allow for steering alignment adjustments to be made.

Ball Joints are protected by rubber boots.

It may only be necessary to replace a cracked or torn tie-rod end ball joint boot instead of the entire joint